Do not leave dangling promises when sending messages over protocol (#742)

It's very bad to have 'unhandled promise rejection' that can't be
handled in user code. These errors will exit node process in a near
future.

This patch avoids 'unhandled promise rejection' while sending protocol
messages.

This patch:
- introduces `puppeteer:error` debug scope and starts using it for all
  swalloed errors.
- makes sure that every `client.send` method is either awaited or its
  errors are handled.
- starts return promises from Request.continue() and Request.abort().
- starts swallow errors from Request.contine() and Request.abort().

The last is the most important part of the patch. Since
`Request.continue()` might try to continue canceled request, we should
disregard the error.

Fixes #627.

lib/FrameManager.js

@@ -16,7 +16,7 @@
 
 const fs = require('fs');
 const EventEmitter = require('events');
-const helper = require('./helper');
+const {helper} = require('./helper');
 const ElementHandle = require('./ElementHandle');
 
 class FrameManager extends EventEmitter {